The present invention relates to a positiveworking photosensitive mixture comprising an o-quinonediazide as the photosensitive compound and a binder of the novolak type, which is insoluble in water and soluble in aqueous-alkaline media.
Due to the continued miniaturization in semiconductor technology, photosensitive mixtures have to satisfy increasing demands in respect of both image resolution and processing methods. Because of their better resolution, positive-working mixtures have been able to gain more and more importance over negative-working mixtures. It is also observed that the wet isotropic etching processes, which lead to undercuttings, are increasingly ousted by anisotropic etching processes, such as plasma etching and reactive ion etching. For these new process methods the quality of the photosensitive mixtures (photoresists) must be further improved, in particular in respect of plasma etch resistance and thermal stability.
Positive-working photosensitive mixtures containing compounds which upon exposure set free an acid, in combination with acid-cleavable compounds, such as acetals and ortho-carboxylates, and relatively high amounts of novolak resins, are known.
Photosensitive mixtures of this type are, for example, described in EP-A-0,022,571; 0,006,626; 0,006,627 and 0,202,196; in DE-A-2,610,842; 3,544,165 and 3,601,264; in DE-C-2,306,248 and 2,718,254 and in German Patent Applications P 3,730,783.5 and P 3,730,785.1
However, most of the prior art publications describe photosensitive mixtures which contain an o-quinone-diazide as the photosensitive compound and a resin of the phenol or cresol novolak type as the binder.
In the patent literature numerous examples are disclosed of how the photolithographic properties of a photosensitive mixture can be modified by modifying the binder which is soluble in aqueous-alkaline media. The criteria mentioned in general are resolution, resistance to developer, contrast, and photosensitivity.
An increased photosensitivity of positive-working photosensitive mixtures is claimed to be achieved by selecting certain mixture: comprising m-cresol/p-cresol or m-cresol/p-cresol/o-cresol, respectively, (EP-A-0,070,624).
By the selection of certain m-cresol/p-cresol mixtures, high resolution due to relief patterns with well-defined edges is claimed to be accomplished for positive-working photosensitive materials in DE-A-36 26 582.
Well-defined edge lines are also claimed to be achieved employing the positive-working photosensitive mixtures known from DE-A-36 03 372, by selecting a novolak resin comprised of m-cresol/p-cresol in combination with 2,5-xylenol.
Novolak resins comprised of m-cresol/p-cresol mixtures in combination with xylenols and condensed with aldehydes are also described in JP-A-16 4740/1985. This publication claims an improved resistance to developer and high photosensitivity for the photosensitive positive-working mixtures disclosed therein.
However, none of the resists prepared from the positive-working photosensitive mixtures described above is distinguished by good thermal stability.
According to DE-A-26 16 992, thermal stability is said to be improved by higher molecular-weight novolaks comprising mono-alkylated alkyl phenols, the alkyl groups of which are in the m- or p-position relative to phenolic hydroxy groups and thus are bireactive (towards nucleophilic reactants).
It is also claimed that novolak resins contained in positive-working photosensitive mixtures, and comprising m-cresol/formaldehyde resins having molecular weights of between 5,000 and 30,000 exhibit both good thermal stability and high photosensitivity (JP-A-57339/1985). Mixtures of this type possess, however, an unsatisfactory resistance to developer, for example, when metal ion-free developers are employed.
Photosensitive mixtures containing pure m-cresol novolak resins as binder, whereby these novolaks have average molecular weights and a low polydispersity, are also said to possess high thermal stability. It has, however, been observed that layers prepared using these mixtures are severely abraded by metal ion-free developers.
Novolak resins obtained from m-cresol and other alkyl-phenols, of a specific molecular weight distribution, are described in EP-A-0,239,423. They are also claimed to possess high thermal stability and good photosensitivity, as well as good resolution.
The state of the art described above in all cases relates to novolak resins which in addition to m-cresol contain bi- or trireactive phenols as monomer components, since at least bireactive reactive monomers are required apart from the aldehyde or ketone component in order to build up a polycondensate chain.
EP-A-0,239,423 for the first time mentions the use of monoreactive phenols, such as 2,4-or 2,6-dimethylphenol, apart from the known m-cresol, for building up novolak resins. Positive-working photosensitive mixtures wherein these resins are contained as binders show, however, a strong dark erosion rate when metal ion-free developers are used. When the dark erosion rate is too high it is no longer possible to achieve a good differentiation between the exposed and unexposed areas of the recording material, i.e., the contrast achieved by development is reduced.